ELECTRON MICROSCOPY 



radiate the object; (b) a condenser lens to 

 concentrate the beam onto the object and in- 

 corporate an aperture system to control the 

 angular aperture of the irradiating beam; 

 (c) an objective lens and objective aperture 

 followed by one or two projector lenses to 

 form an image of the object on a fluorescent 

 screen or photographic plate magnified usu- 

 ally between 1,000 and 200,000 times. The 

 condenser lens may be a double one to allow 

 the reduction of the area of illumination to 

 minimize heat dissipation at the object. The 

 objective lens must have the minimum spher- 

 ical and chromatic aberration and astigma- 

 tism consistent with meeting the geometric 

 requirement of allowing adequate space for 

 the object holder and objective aperture, 

 and their manipulation. The projector lenses 

 must be designed to give the widest range of 

 magnification possible without image dis- 

 tortion. 



The Electron Gun 



The electron gun used in the electron 

 microscope utilizes a tungsten wire hairpin 

 cathode, an apertured grid and an anode. An 

 example of a typical electrode system is 

 shown in Figure 1. The anode is at ground 

 potential and the cathode at the full 50-100 

 kv accelerating potential. The grid is oper- 

 ated with a negative bias of a few hundred 

 volts with respect to cathode. 



To obtain even barely adequate final im- 

 age intensity at magnifications of 50 to 100,- 

 000 required for high resolution working, the 



requirements for gun performance are strin- 

 gent. Since the illumination beam angle is 

 limited to give optimum resolving power, the 

 main gun requirement is to give the maxi- 

 mum possible current density per unit solid 

 angle of emergent beam. Langmuir (1937) 

 has shown that a theoretical limit to the cur- 

 rent density per unit solid angle (or "bright- 

 ness") is imposed by the spread in the emis- 

 sion velocities of the electrons from the 

 cathode. The limiting value of brightness 

 (iS) is given by: 



(3 = pc<j>o/TrkT 



a) 



where pc is the emission current densitj^, 

 cpo the accelerating voltage, k = Boltzman's 

 constant (8.6 X 10"^ e.V./°K) and T the 

 absolute temperature. 



It can readily be shown that the current 

 density obtainable in a focused image of the 

 virtual gun source is independent of the mag- 

 nification of the focusing system where the 

 beam angle is fixed. Haine and Einstein 

 (1952) have shown that the electron micro- 

 scope gun will give this theoretical bright- 

 ness, for a wide range of geometrical con- 

 figurations of the electrodes, provided op- 

 timum bias conditions are maintained. A 

 certain choice of geometrical configurations 

 will give a narrower angle of divergence of 

 the beam and hence conserve total current. 

 Thus, increase of the cathode shield diame- 

 ter and reduction of the height of the cathode 

 behind the shield both reduce beam angle 

 and therefore total current. Under such con- 



^^ 



ATHODE 



SHIELD 



7^ 

 / / 



NODE 





(a) FLAT SHIELD 



Cb") RE-ENTRANT SHELD 



Fig. 1. The typical geometry of an electron gun. 



148 



